1. Technical Field
The present invention relates to a hologram recording apparatus, a hologram recording method and an optical recording medium, and particularly relates to a hologram recording apparatus, hologram recording method and optical recording medium in which a signal that represents digital data as a light and dark image is Fourier-transformed and the Fourier transform image is recorded as a hologram.
2. Related Art
In holographic data storage, zeros and ones of binary digital data are converted to a digital image (signal light) as light pixels and dark pixels, and the signal light is Fourier-transformed by a lens and projected onto an optical recording medium. Thus, the Fourier transform image is recorded as a hologram in the optical recording medium. However, a Fourier transform image of digital data has a very strong peak intensity at zero order. Thus, in holographic data storage, a dynamic range of the optical recording medium is wasted by the zero order optical component (a not scattered component), and there is a problem in that an SNR (signal-to-noise ratio) falls.
In order to solve this problem, a method has been proposed for making a power spectrum of a Fourier transform image uniform, by utilizing a random phase mask. In this method, a random phase mask constituted by a transparent plate which is formed with minute irregularities using photolithography or the like, is used, and a random phase distribution is superimposed on the signal light intensity distribution. Thus, the zero-order component of the signal light is shifted toward the high frequency side, and the intensity distribution of the Fourier transform image to be recorded is made uniform.
However, when the random phase distribution is superimposed on the signal light, there is a problem such that complex speckle noise occurs in a reproduced image so that the SNR of reproduced data decreases. Furthermore, when the random phase distribution is superimposed on the signal light, there is another problem such that the focal depth decreases and there is tight accuracy required for positioning of a light-receiving element at the time of reproduction. A further problem is such that difficulties are encountered in fabricating random phase masks cheaply and in large volumes, and fabrication costs of recording/reproducing apparatuses are increased due to use of random phase masks.